A ventilating apparatus with a control valve for driving a ventilating valve is disclosed in published German Patent Application DT 25 25 359 A1.
In this known ventilating apparatus, the gas necessary for the ventilation flows from the fresh gas metering unit of the ventilating apparatus into a breathing gas hose system which is connected to the patient via a Y-piece. The expiration branch of the hose system is connected to a controllable expiration valve.
The expiration valve is closed during the inspiratory phase so that the ventilating pressure during inhalation can build up in the hose system. However, in the expiration phase, the valve opens and the gas expired by the patient can flow out to the ambient. The control of the expiration valve is achieved with a bistable logic component which serves as a closure element, which, in rhythmic spacing, charges, during the inspiration phase, the control chamber of the expiration valve with a control pressure which closes the valve. For the expiration phase, a switchover to a second lower pressure level occurs by means of which a so-called PEEP-condition is generated in the hose system. PEEP is an acronym which stands for positive end expiratory pressure.
The two pressure levels are adjusted via two switchable branches generating a control pressure via the bistable logic component. Each branch includes a through-flow limiter and a flow throttle. The control gas flowing through the particular through-flow limiter flows off to the atmosphere through the outflow chamber in the closure element and the flow throttle with the developing backpressure functioning as a control pressure on the control chamber of the expiration valve. With the through-flow limiters, the inflowing control gas flow and thereby the control pressure can be varied. The control pressure for the inspiration phase and the expiration phase can be adjusted separately with the through-flow limiters. The bistable logic component in the form of a closure element and the branches generating the control pressure conjointly define the control valve of the expiration valve.
The known expiration valve having the control valve will be referred to in the disclosure which follows as the "expiration valve". These expiration valves have the disadvantage that the least adjustable expiratory pressure is the atmospheric pressure since the control gas can only be released to the atmosphere. The control pressure adjustable at the control valve and thereby the reference pressure present in the control chamber of the expiration valve can be dropped only to the ambient atmosphere as a minimum. However, since the breathing gas encounters a dynamic resistance when flowing through the expiration branch of the breathing gas line, there still always remains the dynamic flow resistance to be overcome in addition to the static reference pressure in the control chamber of the expiratory valve referred to the ambient atmosphere. Thus, this so-called dynamic end expiratory overpressure remains as the breathing gas pressure which must yet be overcome provided that the PEEP at the expiration apparatus was adjusted to zero. In this way, an additional dynamic PEEP pressure is present which has not been calibrated into the entire breathing system even in the presence of a pregiven adjustable PEEP pressure. The PEEP pressure can be different in accordance with the configuration of the expiration apparatus. For this reason, uncontrollable discrepancies can occur between the adjusted PEEP pressure and the actual PEEP pressure.